The invention relates generally to polymer composites. More specifically, the invention relates to a composite having liquid metal inclusions dispersed within an elastic polymer, where the composite has electrical conductivity, permittivity, and thermal conductivity properties that differ from other elastomeric materials.
High compliance (low E) and elasticity (high εf) represent key challenges in the development of mechanically robust electronics that are compatible with natural human tissue for wearable computing, implantable devices, and physical human-machine interaction. Presently, soft and stretchable functionality can be introduced through so-called “deterministic” architectures in which thin flexible circuits are bonded to an elastomer substrate and buckled or patterned into wavy/serpentine planar geometries. Alternatively, solid metal wiring can be replaced with metal alloys that are liquid at room temperature. Liquid-phase gallium-indium alloys are popular for this “microfluidics” approach to stretchable electronics since they are non-toxic and form an oxide layer that aids in microcontact printing, electrode alignment, and 3D printing. Lastly, a common approach to stretchable electronics is to embed thin films of elastomer with a percolating network of conductive particulates, such as structured carbon black or silver nanoparticles, for example. However, their volumetric conductivity is poor compared to bulk conductors and improving conductivity by increasing the weight % of conductive filler increases the mechanical stiffness and brittleness of the material.
Soft electronic systems capable of dielectric behavior are also needed for new technologies such as wearable devices. Traditionally, the electronic properties of rubbery polymers like silicones, polyurethanes, or copolymers such as styrene ethylene butylene styrene are tailored by adding 10-30% by volume of inorganic fillers such as Ag powder, Ag-coated Ni microspheres, structured carbon black (CB), exfoliated graphite, carbon nanotubes, BaTiO3, TiO2, or other metallic, carbon-based, or ceramic micro/nanoparticles. Although rigid particles have been incorporated into silicones, urethanes, and acrylate-based elastomers to increase their dielectric constant, the loadings required to achieve significant electric property enhancement can degrade the mechanical properties of these soft and stretchable material systems. The inherently rigid nature of the inorganic filler particles creates a dramatic compliance mismatch with the soft, stretchable elastomer matrix that leads to internal stress concentrations, delamination, and friction that increase bulk rigidity, reduces extensibility, and results in inelastic stress-strain responses that can limit long term durability at the mesoscale. Other approaches have used fluid fillers to modify mechanical properties, but the electrical properties were not enhanced.
Similarly, efficient thermal transport is critical for applications involving soft electronics. However, heat transport within soft materials is limited by the dynamics of phonon transport, which results in a trade-off between thermal conductivity and compliance. For example, materials with high thermal conductivity are typically rigid and elastically incompatible with soft and mechanically deformable systems. In the general case of non-metallic and electrically insulating solids, this limitation arises from kinetic theory and the Newton-Laplace equation, which imply that thermal conductivity (k) will increase with a material's elastic modulus (E). For polymers like polyethylene, thermal conductivity can be enhanced through macromolecular chain alignment (from k˜0.3 to 100 W/m·K), but this also leads to a dramatic increase in elastic modulus from approximately 1 to 200 GPa. Likewise, glassy polymer blends have been engineered to increase thermal conductivity through interchain hydrogen bonding and relatively higher thermal conductivity has been observed in amorphous polythiophene (k˜4.4 W/m·K), but the high elastic modulus (E˜3 GPa) and low strain at failure (<5% strain) of films make them unsuitable for soft functional materials.
To overcome this fundamental tradeoff with thermal transport in soft materials, attempts have been made to engineer composites with various fillers, including metals, ceramics, carbon fibers, and nanomaterials such as carbon nanotubes and graphene. Although these composites exhibit increased thermal conductivity, they typically utilize rigid fillers that result in mechanically stiff materials that cannot support stretchable functionality and in the case of carbon based fillers, become electrically conductive even at low volume loadings, which can interfere with functionality. To date, the combination of low elastic modulus on the order of biological tissue, large mechanical deformability, and high thermal conductivity remains elusive.
It would therefore be advantageous to develop an elastomeric material with enhanced electrical and thermal properties, yet remains flexible.